How do I print a python slice as a string separated by ":"?

Question

I have a slice object, and I'd like to print it out in the form of a string start:stop:step. How do I do so?

Answer 1

There's nothing built-in to do this, as far as I know, but we can easily write some code to do it using the values from the slice object, per the docs:

Slice objects have read-only data attributes start, stop, and step which merely return the argument values (or their default).

At the most basic

def slice_to_str(s: slice) -> str:
    return f'{s.start!r}:{s.stop!r}:{s.step!r}'

For example:

>>> print(slice_to_str(slice(15, 95, 5)))
15:95:5
>>> print(slice_to_str(slice(60)))
None:60:None

If you want to remove Nones

You can use ... is not None else ''

def slice_to_str(s: slice) -> str:
    start, stop, step = [
        (repr(val) if val is not None else '')
        for val in [s.start, s.stop, s.step]]
    return f'{start}:{stop}:{step}'

For example:

>>> print(slice_to_str(slice(60)))
:60:
>>> print(slice_to_str(slice(None)))
::
>>> print(slice_to_str(slice(None, None, 2)))
::2
>>> print(slice_to_str(slice('h', 't')))
'h':'t':

If you want to remove the last colon when the step is None

Just add a check for it: if s.step is None ...

def slice_to_str(s: slice) -> str:
    start, stop, step = [
        (repr(val) if val is not None else '')
        for val in [s.start, s.stop, s.step]]
    step_optional = f':{step}' if s.step is not None else ''
    return f'{start}:{stop}{step_optional}'

For example:

>>> print(slice_to_str(slice(60)))
:60
>>> print(slice_to_str(slice(None)))
:
>>> print(slice_to_str(slice(None, None, 2)))
::2